This invention relates to a process for the trimerization of isocyanates in the presence of a thermally active catalyst. The catalyst system comprises A) a lithium compound selected from the group consisting of 1) lithium salts of aliphatic or aromatic mono- or dicarboxylic acids, 2) lithium salts of hydroxyl group containing compounds having from 1 to 3 hydroxyl groups per compound, wherein the hydroxyl groups are directly attached to an aromatic ring, and 3) lithium hydroxide; used in conjunction with B) an organic compound containing at least one hydroxyl group.
The trimerization of isocyanates to form polyisocyanurates is well known in the art. Trimerization catalysts described in the prior art include alkali carboxylates as described in DE-OS 3,219,608, basic alkali metal salts complexed with acyclic organic compounds as described in U.S. Pat. No. 4,379,905, basic alkali metal salts complexed with crown ethers as described in U.S. Pat. No. 4,487,928, and combinations of tertiary amines with specific quaternary ammonium salts as described in U.S. Pat. No. 3,954,684.
Catalysts described in U.S. Pat. Nos. 4,632,785 and 4,540,781 comprise alkali metal salts or quaternary ammonium salts of carboxylic acids of the formulas ##STR1## wherein R.sup.1 is alkyl having from 2 to 8 carbon atoms, R.sup.2 is a highly branched alkyl having from 3 to 8 carbon atoms, R.sup.3 is selected from the group consisting of hydrogen, alkyl, and aryl, R.sup.4 is selected from the group consisting of alkyl, aryl, aralkyl, and cycloalkyl, R.sup.5 is independently selected from aryl, and M.sup.+ is a cation selected from the group consisting of alkali metal cations and quaternary ammonium cations of a specific formula.